Gas gauge proximity sensor with internal gas flow control

ABSTRACT

A choked-flow orifice gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff is disclosed. Unlike existing proximity sensors, the gas gauge proximity sensor of the present invention replaces the use of a mass flow controller with a choked flow orifice. The use of a choked flow orifice provides for reduced equipment cost and improved system reliability. A gas supply forces gas into the proximity sensor. The gas is forced through the choked flow orifice to achieve sonic conditions at which time the mass flow rate becomes largely independent of pressure variations. The flow of gas proceeds from the choked flow orifice into a sensor channel system. A mass flow sensor within the sensor channel system monitors flow rates to detect measurement standoffs that can be used to initiate a control action.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing.

2. Background Information

Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.

The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.

Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in photolithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.

U.S. Pat. No. 4,953,388, entitled Air Gauge Sensor, issued Sep. 4, 1990 to Andrew Barada (“'388 patent”), and U.S. Pat. No. 4,550,592, entitled Pneumatic Gauging Circuit, issued Nov. 5, 1985 to Michel Deschape (“'592 patent”), disclose an alternative approach to proximity sensing that uses an air gauge sensor. The '388 and '592 patents are incorporated herein by reference in their entireties. These sensors use reference and measurement nozzles to emit an air flow onto reference and measurement surfaces and measure back pressure differences within the sensors to determine the distance between the measurement nozzle and the measurement surface.

Furthermore, principles of pneumatic gauging are discussed in Burrows, V. R., The Principles and Applications of Pneumatic Gauging, FWP Journal, October 1976, pp. 31-42, which is incorporated herein by reference in its entirety. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers. Earlier versions of air gauge sensors, however, often do not meet today's lithography requirements for precision.

One improvement that has been made to improve the precision of gas gauge proximity sensors is to ensure a stable flow from a gas supply by using a mass flow controller. Mass flow controllers are expensive, often costing over a few thousand dollars. Additionally, to meet the stability requirements of a gas gauge proximity sensor, an expensive precision gas pressure regulator is required at the input to the mass flow controller. The mass flow controller dissipates hear and is mounted remotely from the gas gauge with a supply tube between it and the gas gauge. The supply tube is subject to leaks. Although the leaks are often very small and would not normally affect other types of systems, they can have a significant impact on gas gauge proximity sensor performance. Furthermore, the volume of the tube acts like a capacitor, such that local pressure changes around the gas gauge proximity sensor cause the flow through it to change and several seconds are required for the flow to stabilize.

What are needed are systems and methods for providing more stable operation of a gas gauge proximity sensor.

SUMMARY OF THE INVENTION

The present invention provides gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff. Unlike existing proximity sensors, the gas gauge proximity sensor of the present invention replaces the use of a mass flow controller with a choked flow orifice. The use of a choked flow orifice provides for reduced equipment cost and improved system reliability. A gas supply forces gas into the proximity sensor. The gas is forced through the choked flow orifice to achieve sonic conditions at which time the mass flow rate becomes largely independent of pressure variations. The flow of gas proceeds from the choked flow orifice into a sensor channel system. The sensor channel system includes a measurement and reference channel each having a restrictive element in the channels. The gas flows out of nozzles at the ends of the measurement and reference channels to impinge on a measurement and reference surface, respectively. A bridge channel couples the reference and measurement channels. A mass flow sensor along the bridge channel monitors flow rates to detect measurement standoffs that can be used to initiate a control action.

Further embodiments, features, and advantages of the invention, as well as the structure and operation of the various embodiments of the invention are described in detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.

FIG. 1 is a diagram of a proximity sensor.

FIG. 2 is a diagram of a gas proximity sensor with a choke flow orifice, according to an embodiment of the invention.

FIG. 3 is a diagram of a choke flow orifice.

FIG. 4 is a flowchart of a method to detect very small distances using a choke flow orifice-based gas gauge proximity sensor, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.

Co-pending, commonly owned U.S. patent application Ser. No. 10/322,768, entitled High Resolution Gas Gauge Proximity Sensor, filed Dec. 19, 2002 by Gajdeczko et al., (“'768 patent Application”) describes a high precision gas gauge proximity sensor that overcomes some of the precision limitations of earlier air gauge proximity sensors. The precision limitations are overcome by the introduction of porous snubbers to reduce turbulence in the flow of gases and thereby increase precision. The '768 patent Application, which is incorporated herein by reference in its entirety, describes a gas gauge proximity sensor that provides a high degree of accuracy.

Co-pending, commonly owned U.S. patent application Ser. No. 10/646,720, entitled High Resolution Gas Gauge Proximity Sensor, filed Aug. 25, 2003, by Joseph Lyons, (“'720 patent Application”), describes a proximity sensor in which a specialized nozzle is used to further increase precision and eliminate areas of insensitivity on a measurement surface during measurement operation. The '720 patent Application is incorporated herein by reference in its entirety.

External acoustical interference can also impact gas gauge proximity sensors. Co-pending, commonly owned U.S. patent application Ser. No. 10/854,429 entitled Gas Gauge Proximity Sensor with a Modulated Gas Flow, filed May 27, 2004, by Ebert et al., (“'429 patent Application”) describes a gas gauge proximity sensor that modulates a gas stream at a modulated frequency in which there is minimal acoustical interference energy, thereby improving measurement precision. The '429 patent Application is incorporated herein by reference in its entirety.

While the sensors disclosed in the '768, '720, and '429 patent applications provide a high degree of precision, the precision can be impacted by changes in local environmental conditions near measurement and reference nozzles. In one circumstance, even though the nozzles are often very close together minor differences in environmental conditions can impact sensor accuracy. Co-pending, commonly owned U.S. patent application Ser. No. 10/833,249 entitled High Resolution Gas Gauge Proximity Sensor, filed Apr. 28, 2004, by Carter et al., (“'249 patent Application”) describes a gas gauge proximity sensor that includes a chamber that reduces environmental differences across measurement and references nozzles. The '249 patent Application is incorporated herein by reference in its entirety.

A similar problem relates to cross flows of gas or liquid that intersect the stream of gas or liquid that is being emitted from a measurement channel of the proximity sensor. Specifically, purging gases, for example, can exhibit local cross winds with velocities of the order of a few meters per second. Cross-winds or cross-flows will cause gauge instability and drift, introducing non-calibratable errors within proximity sensors. Co-Pending, commonly owned U.S. patent application Ser. No. 11/005,246, entitled Proximity Sensor Nozzle Shroud with Flow Curtain, filed Dec. 7, 2004, by Herman Vogel (“'246 patent Application”) describes a proximity sensor that includes a shroud around the nozzles to reduce the impact on cross winds. The '246 patent Application is incorporated herein by reference in its entirety.

Proximity sensors must be non-intrusive. Contact between a proximity sensor and a work surface can significantly degrade or ruin the semiconductor quality of quality of other work surface. However, to ensure the greatest level of precision often the measurement nozzle must be extremely close to the work surface. In certain circumstances, as higher levels of precision are required, the movement of a wafer stage or other work platform is such that it is desirable to move a proximity sensor toward and away from a work surface. This leads to another source of imprecision related to the mechanical stability of a proximity sensor head, when it is moved up and down. When the sensor head is extended it can drift thereby reducing the accuracy of the proximity sensor. Co-pending, commonly owned, U.S. patent application Ser. No. 11/015,652 entitled Proximity Sensor with Self Compensation for Mechanism Instability, filed Dec. 20, 2004, by Peter Kochersperger, (“'652 patent Application”) discloses a retractable proximity sensor that includes a self compensating mechanism to reduce the impact of proximity sensor head drift on the accuracy of the proximity sensor. The '652 patent Application is incorporated herein by reference in its entirety.

FIG. 1 provides a diagram of gas gauge proximity sensor 100. Gas gauge proximity sensor 100 is one type of proximity sensor that can be improved through use of the present invention, and is not intended to limit the scope of the invention. Gas gauge proximity sensor 100 includes gas pressure regulator 105, mass flow controller 106, central channel 112, measurement channel 116, reference channel 118, measurement channel restrictor 120, reference channel restrictor 122, measurement probe 128, reference probe 130, bridge channel 136 and mass flow sensor 138. Gas supply 102 injects gas at a desired pressure into gas gauge proximity sensor 100.

Central channel 112 connects gas supply 102 to gas pressure regulator 105 and mass flow controller 106 and then terminates at junction 114. Gas pressure regulator 105 and mass flow controller 106 maintains a constant flow rate within gas gauge proximity sensor 100.

Gas is forced out from mass flow controller 106 into channel 112 with an accumulator 108 affixed to channel 112. In some situations, a snubber, which is not shown in the diagram, can be placed between mass flow controller 106 and junction 114. A snubber reduces gas turbulence introduced by the gas supply 102. A more complete description of snubber 110 can be found in the '249 patent Application. Upon exiting mass flow controller 106, gas travels through central channel 112 to junction 114. Central channel 112 terminates at junction 114 and divides into measurement channel 116 and reference channel 118. Mass flow controller 106 injects gas at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired pneumatic noise. Likewise, the system geometry can be appropriately sized to maintain the laminar flow characteristics established by mass flow controller 106.

As is discussed above, mass flow controllers, such as mass flow controller 106, are expensive and often cost over a few thousand dollars. Mass flow controller 106 dissipates heat and is mounted remotely from the other components of gas gauge proximity sensor 100. Pliable supply tubes couple mass flow controller 106 to junction 114. The supply tube is subject to leaks. Although the leaks are often very small and would not normally affect other types of systems, they can have a significant impact on gas gauge proximity sensor performance. Furthermore, the volume of the tube acts like a capacitor, such that local pressure changes around the gas gauge proximity sensor cause the flow through it to change and several seconds are required for the flow to stabilize. As is discussed below, the present invention addresses these operational challenges.

Bridge channel 136 is coupled between measurement channel 116 and reference channel 118. Bridge channel 136 connects to measurement channel 116 at junction 124. Bridge channel 136 connects to reference channel 118 at junction 126. In one example, the distance between junction 114 and junction 124 and the distance between junction 114 and junction 126 are equal.

All channels within gas gauge proximity sensor 100 permit gas to flow through them. Channels 112, 116, 118, and 136 can be made up of conduits (tubes, pipes, etc.) or any other type of structure that can contain and guide gas flow through sensor 100. It is preferred that the channels do not have sharp bends, irregularities or unnecessary obstructions that may introduce pneumatic noise, for example, by producing local turbulence or flow instability. The overall lengths of measurement channel 116 and reference channel 118 can be equal or in other examples can be unequal.

Reference channel 118 terminates into reference nozzle 130. Likewise, measurement channel 116 terminates into measurement nozzle 128. Reference nozzle 130 is positioned above reference surface 134. Measurement nozzle 128 is positioned above measurement surface 132. In the context of photolithography, measurement surface 132 is often a semiconductor wafer, stage supporting a wafer, flat panel display, a print head, a micro- or nanofluidic device or the like. Reference surface 134 can be a flat metal plate, but is not limited to this example. Gas injected by gas supply 102 is emitted from each of the nozzles 128, 130 and impinges upon measurement surface 132 and reference surface 134. As stated above, the distance between a nozzle and a corresponding measurement or reference surface is referred to as a standoff.

Measurement channel restrictor 120 and reference channel restrictor 122 serve to reduce turbulence within the channels and act as a resistive element. In other embodiments, other types of resistive elements, such as, orifices can be used. Although orifices will not reduce turbulence.

In one embodiment, reference nozzle 130 is positioned above a fixed reference surface 134 with a known reference standoff 142. Measurement nozzle 128 is positioned above measurement surface 132 with an unknown measurement standoff 140. The known reference standoff 142 is set to a desired constant value representing an optimum standoff. With such an arrangement, the backpressure upstream of the measurement nozzle 128 is a function of the unknown measurement standoff 140; and the backpressure upstream of the reference nozzle 130 is a function of the known reference standoff 142. If standoffs 140 and 142 are equal, the configuration is symmetrical and the bridge is balanced. Consequently, there is no gas flow through bridging channel 136. On the other hand, when the measurement standoff 140 and reference standoff 142 are different, the resulting pressure difference between the measurement channel 116 and the reference channel 118 induces a flow of gas through mass flow sensor 138.

Mass flow sensor 138 is located along bridge channel 136, preferably at a central location. Mass flow sensor 136 senses gas flows induced by pressure differences between measurement channel 116 and reference channel 118. These pressure differences occur as a result of changes in the vertical positioning of measurement surface 132. For a symmetric bridge, when measurement standoff 140 and reference standoff 142 are equal, the standoff is the same for both of the nozzles 128, 130 compared to surfaces 132, 134. Mass flow sensor 138 will detect no mass flow, since there will be no pressure difference between the measurement and reference channels. Differences between measurement standoff 140 and reference standoff 142 will lead to different pressures in measurement channel 116 and reference channel 118. Proper offsets can be introduced for an asymmetric arrangement.

Mass flow sensor 138 senses gas flow induced by a pressure difference or imbalance. A pressure difference causes a gas flow, the rate of which is a unique function of the measurement standoff 140. In other words, assuming a constant flow rate into gas gauge 100, the difference between gas pressures in the measurement channel 116 and the reference channel 118 is a function of the difference between the magnitudes of standoffs 140 and 142. If reference standoff 142 is set to a known standoff, the difference between gas pressures in the measurement channel 116 and the reference channel 118 is a function of the size of measurement standoff 140 (that is, the unknown standoff between measurement surface 132 and measurement nozzle 128).

Mass flow sensor 138 detects gas flow in either direction through bridge channel 136. Because of the bridge configuration, gas flow occurs through bridge channel 136 only when pressure differences between channels 116, 118 occur. When a pressure imbalance exists, mass flow sensor 138 detects a resulting gas flow, and can initiate an appropriate control function. Mass flow sensor 138 can provide an indication of a sensed flow through a visual display, audio indication, computer controlled system or other signaling means. Alternatively, in place of a mass flow sensor, a differential pressure sensor may be used. The differential pressure sensor measures the difference in pressure between the two channels, which is a function of the difference between the measurement and reference standoffs.

Proximity sensor 100 is provided as one example of a device with a nozzle that can benefit from the present invention. The invention is not intended to be limited to use with only proximity sensor 100. Rather the invention can be used to improve other types of proximity sensors, such as, for example, the proximity sensors disclosed in the '388 and '592 patent, and the '768, '720, '429, '249, '286, and '652 patent Applications.

FIG. 2 is a diagram of proximity sensor 200, according to an embodiment of the invention. The primary difference between proximity sensor 200 and proximity sensor 100 is the replacement of mass flow controller 106 with a choked flow orifice 207. Gas gauge proximity sensor 200 includes gas pressure regulator 205, choked flow orifice 207, central channel 212, measurement channel 216, reference channel 218, measurement channel restrictor 220, reference channel restrictor 222, measurement probe 228, reference probe 230, bridge channel 236 and mass flow sensor 238. Gas supply 202 injects gas at a desired pressure into gas gauge proximity sensor 200.

Central channel 212 connects gas supply 202 to gas pressure regulator 205 and choked flow orifice 207 and then terminates at junction 214. Gas pressure regulator 205 and choked flow orifice 207 maintain a constant flow rate within gas gauge proximity sensor 200. Gas is forced out from choked flow orifice 207 into channel 212. In some situations, a snubber, which is not shown in the diagram, can be placed between choked flow orifice 207 and junction 214. Upon exiting choked flow orifice 207, gas travels through central channel 212 to junction 214.

Choked flow orifice 207 delivers the desired flow under choked flow conditions at a reasonable supply pressure. At choked flow, the mass flow through choked flow orifice 207 becomes theoretically independent of pressure differences. The volume between choked flow orifice 207 and restrictors 220 and 222 is several orders of magnitude smaller than the volume of current supply tubes used, for example, with proximity sensor 100, so that proximity sensor 200 responds to local pressure variations much faster than proximity sensor 100.

As a result, the overall performance of gas gauge proximity sensor 200 becomes more stable than the operation of proximity sensor 100. Additionally, small leaks in the supply tube forming channel 112 into choked flow orifice 207 will not impact the pressure and will have limited to no impact on the flow through the choked flow orifice 207, thus the reliability of proximity sensor 200 will be high. The end result is that the use of choked flow orifice 207 results in a more stable sensor, which is cheaper to produce because the mass flow controller is eliminated and more reliable because there is less risk of leaks effecting performance.

Choked flow orifices, such as choked flow orifice 207 are available from specialty controls companies. For example, O'Keefe Control Co. of Trumbull, Conn. provides a sapphire orifice with integral wire screen that can be used. The operation of choked flow orifice 207 is illustrated in FIG. 3. In gas flow through choked flow orifice 207, when the ratio of the downstream pressure, P₂, to the upstream pressure, P₁, is less than or equal to about 0.528, sonic conditions exist. When sonic conditions exist, the gas velocity exiting choked flow orifice 207 is such that is remains constant for all pressure ratios less than about 0.528. As a result, the use of the choked flow orifice 207 isolates minor drops in pressure that may occur from a small leak and provides a steady flow of gas through the channel structure of the proximity sensor.

Following the choked flow orifice 207, the architecture of the proximity sensor 200 is the same as that of proximity sensor 100. The portion of the architecture that follows choked flow orifice 207 can be referred to as the sensing channel system. The sensing channel system described below is example of a sensing channel system that can be used with a gas supply and choked flow orifice to form a proximity sensor. Other sensing channel systems such as, for example, those disclosed in the '388 and '592 patents, and the '768, '720, '429, '249, '246 and '652 patent Applications. The remaining architecture of proximity sensor 200 is provided here for completeness.

Central channel 212 terminates at junction 214 and divides into measurement channel 216 and reference channel 218. Choked flow orifice 207 injects gas at a sufficiently low rate to provide laminar and incompressible fluid flow throughout the system to minimize the production of undesired pneumatic noise. Likewise, the system geometry can be appropriately sized to maintain the laminar flow characteristics established by choked flow orifice 207.

Bridge channel 236 is coupled between measurement channel 216 and reference channel 218. Bridge channel 236 connects to measurement channel 216 at junction 224. Bridge channel 236 connects to reference channel 218 at junction 226. In one example, the distance between junction 214 and junction 224 and the distance between junction 214 and junction 226 are equal. Channels 216 and 218 include channel restrictors 220 and 222 respectfully, which are described in detail below.

All channels within gas gauge proximity sensor 200 permit gas to flow through them. Channels 212, 216, 218, and 236 can be made up of conduits (tubes, pipes, etc.) or any other type of structure that can contain and guide gas flow through sensor 200. It is preferred that the channels do not have sharp bends, irregularities or unnecessary obstructions that may introduce pneumatic noise, for example, by producing local turbulence or flow instability. The overall lengths of measurement channel 216 and reference channel 218 can be equal or in other examples can be unequal.

Reference channel 218 terminates into reference nozzle 230. Likewise, measurement channel 216 terminates into measurement nozzle 228. Reference nozzle 230 is positioned above reference surface 234. Measurement nozzle 228 is positioned above measurement surface 232. In the context of photolithography, measurement surface 232 is often a semiconductor wafer, stage supporting a wafer, flat panel display, a print head, a micro- or nanofluidic device or the like. Reference surface 234 can be a flat metal plate, but is not limited to this example. Gas injected by gas supply 202 is emitted from each of the nozzles 228, 230 and impinges upon measurement surface 232 and reference surface 234. As stated above, the distance between a nozzle and a corresponding measurement or reference surface is referred to as a standoff.

Measurement channel restrictor 220 and reference channel restrictor 222 serve to reduce turbulence within the channels and act as a resistive element. In other embodiments, other types of resistive elements, such as, orifices can be used. Although orifices will not reduce turbulence.

In one embodiment, reference nozzle 230 is positioned above a fixed reference surface 234 with a known reference standoff 242. Measurement nozzle 228 is positioned above measurement surface 232 with an unknown measurement standoff 240. The known reference standoff 242 is set to a desired constant value representing an optimum standoff. With such an arrangement, the backpressure upstream of the measurement nozzle 228 is a function of the unknown measurement standoff 240; and the backpressure upstream of the reference nozzle 230 is a function of the known reference standoff 242. If standoffs 240 and 242 are equal, the configuration is symmetrical and the bridge is balanced. Consequently, there is no gas flow through bridging channel 236. On the other hand, when the measurement standoff 240 and reference standoff 242 are different, the resulting pressure difference between the measurement channel 216 and the reference channel 218 induces a flow of gas through mass flow sensor 238.

Mass flow sensor 238 is located along bridge channel 236, preferably at a central location. Mass flow sensor 236 senses gas flows induced by pressure differences between measurement channel 216 and reference channel 218. These pressure differences occur as a result of changes in the vertical positioning of measurement surface 232. For a symmetric bridge, when measurement standoff 240 and reference standoff 242 are equal, the standoff is the same for both of the nozzles 228, 230 compared to surfaces 232, 234. Mass flow sensor 238 will detect no mass flow, since there will be no pressure difference between the measurement and reference channels. Differences between measurement standoff 240 and reference standoff 242 will lead to different pressures in measurement channel 216 and reference channel 218. Proper offsets can be introduced for an asymmetric arrangement.

Mass flow sensor 238 senses gas flow induced by a pressure difference or imbalance. A pressure difference causes a gas flow, the rate of which is a unique function of the measurement standoff 240. In other words, assuming a constant flow rate into gas gauge 200, the difference between gas pressures in the measurement channel 216 and the reference channel 218 is a function of the difference between the magnitudes of standoffs 240 and 242. If reference standoff 242 is set to a known standoff, the difference between gas pressures in the measurement channel 216 and the reference channel 218 is a function of the size of measurement standoff 240 (that is, the unknown standoff between measurement surface 232 and measurement nozzle 228).

Mass flow sensor 238 detects gas flow in either direction through bridge channel 236. Because of the bridge configuration, gas flow occurs through bridge channel 236 only when pressure differences between channels 216, 218 occur. When a pressure imbalance exists, mass flow sensor 238 detects a resulting gas flow, and can initiate an appropriate control function. Mass flow sensor 238 can provide an indication of a sensed flow through a visual display or audio indication. Alternatively, in place of a mass flow sensor, a differential pressure sensor may be used. The differential pressure sensor measures the difference in pressure between the two channels, which is a function of the difference between the measurement and reference standoffs.

Proximity sensor 200 is provided as one example of a device with a nozzle that can benefit from the present invention. The invention is not intended to be limited to use with only proximity sensor 200. Rather the introduction of a choked flow orifice to replace a mass flow controller can be used to improve other types of proximity sensors, such as, for example, the proximity sensors disclosed in the '388 and '592 patents, and the '768, '720, '429, '249, '286, and '652 patent Applications.

FIG. 4 provides a flowchart of a method 400 for using gas flow to detect very small distances and perform a control action. For convenience, method 400 is described with respect to gas gauge proximity sensor 200. However, method 400 is not necessarily limited by the structure of sensor 200, and can be implemented with a sensor having a different structure.

The process begins in step 410. In step 410, an operator or mechanical device places a reference probe above a reference surface. For example, an operator or mechanical device positions reference probe 230 above reference surface 234 with known reference standoff 242. Alternatively, the reference standoff can be arranged within the sensor assembly, that is, internal to the sensor assembly. The reference standoff is pre-adjusted to a particular value, which typically would be maintained constant.

In step 420, an operator or mechanical device places a measurement probe above a measurement surface. For example, an operator or mechanical device positions measurement probe 228 above measurement surface 232 to form measurement gap 240.

In step 430, gas is injected into a sensor. For example, a measurement gas is injected into gas gauge proximity sensor 200 with a constant mass flow rate.

In step 440, gas is forced through a choked flow orifice. For example, gas can be forced through choke flow orifice 207 to achieve sonic conditions at which point the mass flow through the orifice becomes largely independent of pressure differences. A constant gas flow rate into a sensor is maintained. For example, choke flow orifice 207 maintains a constant gas flow rate.

In step 450, gas flow is distributed between measurement and reference channels. For example, gas gauge proximity sensor 200 causes the flow of the measurement gas to be evenly distributed between measurement channel 216 and reference channel 218.

In step 460, gas flow in the measurement channel and the reference channel is restricted evenly across cross-sectional areas of the channels. Measurement channel restrictor 220 and reference channel restrictor 222 restrict the flow of gas to reduce pneumatic noise and serve as a resistive element in gas gauge proximity sensor 200.

In step 470, gas is forced to exit from a reference and measurement probe. For example, gas gauge proximity sensor 200 forces gas to exit measurement probe 228 and reference probe 230. In step 480, a flow of gas is monitored through a bridge channel connecting a reference channel and a measurement channel. In step 490, a control action is performed based on a pressure difference between the reference and measurement channel. For example, mass flow sensor 238 monitors mass flow rate between measurement channel 216 and reference channel 218. Based on the mass flow rate, mass flow sensor 238 initiates a control action. Such control action may include providing an indication of the sensed mass flow, sending a message indicating a sensed mass flow, or initiating a servo control action to reposition the location of the measurement surface relative to the reference surface until no mass flow or a fixed reference value of mass flow is sensed. In step 495, method 400 ends.

CONCLUSION

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.

The present invention has been described above with the aid of method steps illustrating the performance of specified functions and relationships thereof. The boundaries of these method steps have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

The Detailed Description section should primarily be used to interpret the claims. The Summary and Abstract sections may set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit claims. 

1. A gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff, comprising: a gas supply that injects gas into the gas gauge proximity sensor; a choked flow orifice coupled to the gas supply that chokes the flow of gas; and a sensor channel system coupled to the choked flow orifice that detects the difference in standoffs between the reference and measurement surfaces at a high sensitivity.
 2. The gas gauge proximity sensor of claim 1, wherein the sensor channel system includes: a junction that combines gas flow into a channel coupled to the choked flow orifice of the gas gauge proximity sensor, wherein the junction combines a reference channel and a measurement channel; a first resistive element located in the reference channel, wherein said first resistive element evenly restricts gas flow through the reference channel; a second restrictive element located in the measurement channel, wherein said second restrictive element evenly restricts gas flow through the measurement channel; a reference probe at an end of the reference channel, whereby gas enters the reference channel through the reference probe having traveled across the reference surface standoff; a measurement probe at an end of the measurement channel, whereby gas enters the measurement channel through the measurement probe having traveled across a measurement surface standoff; and a mass flow sensor coupled between the reference and measurement channels that senses the mass of gas flow therebetween, whereby the difference in standoffs between the reference and measurement surfaces can be sensed at a high sensitivity.
 3. The gas gauge proximity sensor of claim 2, wherein said first and second restrictive elements comprise a porous restrictor or orifice.
 4. The gas gauge proximity sensor of claim 1, wherein the gas comprises air.
 5. A method for sensing a difference in a reference standoff and a measurement standoff, comprising the steps of: (a) injecting a flow of gas into a proximity sensor having a measurement channel and reference channel; (b) forcing the flow of gas through a choked flow orifice to achieve sonic conditions within the flow of the gas; (c) distributing a flow of gas between a measurement channel and a reference channel; (d) outputting gas from the reference and measurement channels through nozzles to impinge upon a reference surface and a measurement surface, respectively; and (e) sensing a mass flow rate across a bridge channel that connects the reference and measurement channels, the mass flow rate being representative of the magnitude of a difference between a measurement standoff and a reference standoff.
 6. The method of claim 5, wherein step (c) comprises restricting the flow of gas substantially evenly across cross-sectional areas of both the measurement and reference channels.
 7. The method of claim 5, wherein step (e) comprises the step of monitoring the mass flow rate across a bridge channel that connects the reference and measurement channels, the mass flow rate being representative of the magnitude of the difference between the measurement standoff and the reference standoff.
 8. The method as in claim 7, further comprising performing a control action in response to said sensing step.
 9. The method of claim 5, wherein step (e) comprises the step of monitoring gas pressure differences in the reference and measurement channels, the gas pressure differences being representative of the magnitude of the difference between the measurement standoff and the reference standoff.
 10. The method as in claim 9, further comprising performing a control action in response to said sensing step.
 11. The method as in claim 5, further comprising performing a control action in response to said sensing step.
 12. A gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff within a lithography apparatus, comprising: a gas supply that injects gas into the gas gauge proximity sensor; a choked flow orifice coupled to the gas supply that chokes the flow of gas; and a sensor channel system coupled to the choked flow orifice that detects the difference in standoffs between the reference and measurement surfaces at a high sensitivity.
 13. The gas gauge proximity sensor of claim 12, wherein the sensor channel system includes: a junction that combines gas flow into a channel coupled to the choked flow orifice of the gas gauge proximity sensor, wherein the junction combines a reference channel and a measurement channel; a first resistive element located in the reference channel, wherein said first resistive element evenly restricts gas flow through the reference channel; a second restrictive element located in the measurement channel, wherein said second restrictive element evenly restricts gas flow through the measurement channel; a reference probe at an end of the reference channel, whereby gas enters the reference channel through the reference probe having traveled across the reference surface standoff; a measurement probe at an end of the measurement channel, whereby gas enters the measurement channel through the measurement probe having traveled across a measurement surface standoff; and a mass flow sensor coupled between the reference and measurement channels that senses the mass of gas flow therebetween, whereby the difference in standoffs between the reference and measurement surfaces can be sensed at a high sensitivity.
 14. The gas gauge proximity sensor of claim 13, wherein said first and second restrictive elements comprise a porous restrictor or orifice.
 15. The gas gauge proximity sensor of claim 12, wherein the gas comprises air. 